Added index on git_hooks.project_id

On PostgreSQL this index is created concurrently.

Fixes gitlab-org/gitlab-ce#14324

db/migrate/20160316124047_git_hooks_project_id_index.rb

+class GitHooksProjectIdIndex < ActiveRecord::Migration
+  disable_ddl_transaction!
+
+  def change
+    args = [:git_hooks, :project_id]
+
+    if Gitlab::Database.postgresql?
+      args << { algorithm: :concurrently }
+    end
+
+    add_index(*args)
+  end
+end

db/schema.rb

@@ -11,7 +11,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema.define(version: 20160314143402) do
+ActiveRecord::Schema.define(version: 20160316124047) do
 
   # These are extensions that must be enabled in order to support this database
   enable_extension "plpgsql"
@@ -440,6 +440,8 @@ ActiveRecord::Schema.define(version: 20160314143402) do
     t.integer  "max_file_size",        default: 0,     null: false
   end
 
+  add_index "git_hooks", ["project_id"], name: "index_git_hooks_on_project_id", using: :btree
+
   create_table "historical_data", force: :cascade do |t|
     t.date     "date",              null: false
     t.integer  "active_user_count"